The invention generally relates to optical exposure systems for semiconductor wafers and more particularly to an improved system and method that reduces the amount of time needed to align the wafer in the optical exposure system.
With the everspeed of wafer exposure systems, the time spent on alignment is becoming the limiting factor in the total throughput of steppers or stepsystems. Typical time spent for a single wafer alignment can reach as much as 10 seconds, which represents a large portion of the total time that is needed for single wafer exposure. For example, if an exposure process is able to produce 60 wafers per hour, each wafer would take one minute to expose. Similarly, if the exposure process is able to produce 90 wafers per hour, each wafer is exposed in 40 seconds. Therefore, the 10 seconds required to align each wafer becomes a significant factor.
One conventional system attempts to reduce the time required to align the wafer using a “twin stage” system. In such a twin stage system, a single machine includes two wafer stages. While one said wafer stages is being aligned, the other said wafer stages can expose a wafer. This directly eliminates the alignment time; however, such devices are substantially larger (30% in size and weight than the single stage devices and are more expensive than single stage devices. An additional drawback of twin stage systems is that if one of the stages experiences a defect, the machine must stop production, which dramatically increases inefficiency for the single defect.
Therefore, if the savings in alignment time can be achieved using single stage systems, such benefits are produced with fewer drawbacks when compared to twin stage systems. The invention described below reduces the alignment time without having to resort to twin stage systems.